A cell - such as a skin cell - is taken from and adult animal , the nucleus, an the DNA it houses, is sucked out and placed next to an empty egg cell - one that has had its nucleus removed. The new egg-nucleus combo is then jolted with electricity or bathed in a chemical cocktail.

"What you want to do is basically trick the egg into thinking it's been fertilized by a sperm, "says Neal First, a retired professor of animal sciences at the University of Wisconsin - Meadison, and the first researcher to clone cattle.

If all goes well, the duped egg starts to divide, eventually creating an incipient embryo, which researchers implant into a surrogate animal.

Whit this may sound pretty straightforward, it's actually a messy, hit-or-miss process that yields few successful clone. Depending on whom you talk to, the number of successful clones -which survive beyond birth - can run a slow as one-in-1000 to as many as 15 per cent. Researchers believe this is the result of a host of molecular issues some they can pinpoint, others they can't.

They mystery is in the egg, "There are molecules in the egg that allow t he DNA to reprogramme" and start a new - so that it's read as the blueprint for an embryo, not an old skin cell, Lanza says.

But what those molecules are and how they work remains elusive.